An essential role of Bmp4 in the atrioventricular septation of the mouse heart

Abstract

Proper septation and valvulogenesis during cardiogenesis depend on interactions between the myocardium and the endocardium. By combining use of a hypomorphic Bone morphogenetic protein 4 (Bmp4) allele with conditional gene inactivation, we here identify Bmp4 as a signal from the myocardium directly mediating atrioventricular septation. Defects in this process cause one of the most common human congenital heart abnormalities, atrioventricular canal defect (AVCD). The spectrum of defects obtained through altering Bmp4 expression in the myocardium recapitulates the range of AVCDs diagnosed in patients, thus providing a useful genetic model with AVCD as the primary defect. Congenital heart malformation is the most common human birth defect and the leading cause of death in the first year of life (Hoffman 1995). One of the most frequently diagnosed disorders is atrioventricular canal defect (AVCD), which accounts for 7.3% of all congenital heart abnormalities (Pierpont et al. 2000). During normal development, septation of the AV canal (AVC) is initiated with the formation of the inferior and superior endocardial cushions through epithelial-mesenchymaltransformation (EMT) by some endocardial cells invading into cardiac jelly, the extracellular matrix between the endocardium and myocardium (Nakajima et al. 2000;Markwald and Wessels 2001). Subsequent growth and fusion of the AV cushions produce the central mesenchymal mass, which further develops into the mature AV septum and valves through complex remodeling processes. The central mesenchymal mass also fuses with the atrial septum primum (ASP) and the inlet portion of the ventricular septum to prevent abnormal blood flow between chambers (Marino and Digilio 2000; Markwald and Wessels 2001). AVCD covers a spectrum of abnormalities, from the partial form with defects in the lower part of the ASP to the complete form in which absence of the AV septum results in a single common AVC (Marino and Digilio 2000).The complex cushion morphogenesis during AV septation depends both on signals released from the overlying myocardium and on proper responses of the endocardial and mesenchymal cells (Nakajima et al. 2000;Markwald and Wessels 2001). The molecular pathways for the initiation of cushion formation (EMT) have been studied extensively. In both chicken and mouse explant culture assays, transforming growth factor ␤2 (TGF␤2) is able to replace the overlying myocardium to activate EMT, and inhibition of TGF␤2 activity blocks EMT (Boyer et al. 1999;Nakajima et al. 2000;Camenisch et al. 2002a). Consistent with these in vitro studies, TGF␤2-deficient mice show defects in valvulogenesis (Bartram et al. 2001). In addition to the TGF␤ signaling pathway, the Hyaluronic acid (Ha) synthase 2 (Has2)/ErbB2,ErbB3/Ras pathway has been recently shown to play essential roles in EMT (Camenisch et al. 2000(Camenisch et al. , 2002b. Ha is a prominent component of cardiac jelly in embryonic day 9.5 (E9.5) embryos and its production depends on Has2. In addition to serving as a substrate for migrati...

BMP and FGF regulatory pathways in semilunar valve precursor cells

“…BMP and FGF signaling pathways have previously been shown to be important for normal heart development including endocardial cushion formation, cushion elongation, and remodeling (Galvin et al, 2000;Gaussin et al, 2002;Delot et al, 2003;Jiao et al, 2003;Sugi et al, 2003Sugi et al, , 2004Somi et al, 2004;Ma et al, 2005;Lincoln et al, 2006a). A number of BMP ligands have been identified in the developing heart; in particular, BMP4 is expressed in the OFT myocardium adjacent to the endocardial cushions during early stages of valve development (Keyes et al, 2003).…”

“…A number of BMP ligands have been identified in the developing heart; in particular, BMP4 is expressed in the OFT myocardium adjacent to the endocardial cushions during early stages of valve development (Keyes et al, 2003). The importance of BMP signaling during cushion formation is supported by studies identifying early roles for BMP ligand and receptors during EMT and formation of the OFT endocardial cushions, but further studies are required to confirm the in vivo role of BMP in valve precursor cells during valve maturation (Gaussin et al, 2002;Jiao et al, 2003;Ma et al, 2005). As shown for BMP2 in developing AV valve precursor cells (Lincoln et al, 2006a), the present studies show that BMP4 is sufficient to activate Smad 1/5/8 signaling.…”

“…Furthermore, despite inferences from the distribution of BMP ligands and receptors during heart formation (reviewed in Delot, 2003), there have been important obstacles to the study of BMP effects using conventional generation of null embryos, which have often resulted in no cardiac phenotype (presumably due to redundancy) or embryo lethality prior to cushion or valve formation (Delot, 2003;Armstrong and Bischoff, 2004). Recent studies utilizing conditional ablation of ligands (Jiao et al, 2003;Rivera-Feliciano and Tabin, 2006), conditional deletion of the type I receptor, ALK3 (Gaussin et al, 2002(Gaussin et al, , 2005, and generation of hypomorphic alleles of other BMP receptors , have provided new tools for future investigation and important insights into the role of BMP signaling in murine heart valve development.…”

Generation and Characterization of a Tissue‐Specific Centrosome Indicator Mouse Line

“…Protamine‐Cre (O'Gorman et al ., 1997), Nkx2.5‐Cre Troponin T‐Cre (Jiao et al ., 2003), and R26‐mTmG (Muzumdar et al ., 2007) were purchased from Jackson Laboratories. ROSA‐ Neo ‐EGFP‐Cetn1 line have been bred into a Black Swiss outbred background for more than 6 generations, did not show any overt difference from wild‐type mouse.…”

Co-ordinating Notch, BMP, and TGF-β signaling during heart valve development

“…BMP ligands 2, 4, 5, 6, and 7 are all expressed in AVC and OFT myocardium during valve formation [39][40][41][42][43][44]. The BMP type I receptor, ALK2 is expressed in endocardium and a subset of mesenchymal cells in the heart at E10 [45].…”